Ethanol for fuel derived from a plant is expected to be a liquid fuel alternative to gasoline to prevent increase in carbon dioxide gas, and a method for producing ethanol by fermenting sugar liquid derived from a plant with microorganisms has been conventionally investigated. However, there is a problem that consumption of sugar liquid derived from a plant as a raw material for production of ethanol puts pressure on production of sugar, which is food.
As a method for solving this problem, Patent Document describes a method for producing sugar and ethanol wherein a plant-origin sugar solution is fermented by a sucrose unassimilating yeast. According to this method, selective ethanol fermentation of reducing sugar is carried out using a sucrose unassimilating yeast before a sugar crystallization step. By means of this process, improvement in productivity of sugar and ethanol production are simultaneously achieved.
However, there has been conventionally almost no example of that a sucrose unassimilating yeast is utilized in food production, and there are few microbial species of which safety for a human body has been confirmed. Hence a method for producing sugar and ethanol using a sucrose unassimilating yeast has a problem that there are few microbial species which can be used therefor and that room for improvement in the process is limited.
Some sucrose unassimilating yeasts belong to Saccharomyces cerevisiae. Saccharomyces cerevisiae is a yeast strain which has much of a proven performance of food production, and excellent in safety for a human body. However, among the sucrose unassimilating yeasts belonging to Saccharomyces cerevisiae, a yeast which is excellent in fermentability is non-flocculent and hardly sediments. Therefore, troublesome operations such as centrifugation and microfiltration are required for removing the yeast from a sugar liquid after fermentation.
Here, among the sucrose unassimilating yeasts belonging to Saccharomyces cerevisiae, a yeast which exhibits flocculation (STX 347-1D strain) is auxotrophic, and has a problem that fermentation hardly progresses in a case where uracil (a base) and histidine (an amino acid) are not present in a medium.
Therefore, as for a method for producing sugar and ethanol using a sucrose unassimilating yeast, it is still difficult to expand the production scale and reduce production cost, while securing production conditions excellent in safety and practicality.